How large is the magma chamber that is currently under Yellowstone?
The magma chamber is believed to be about 40 by 80 kilometers across, similar in size to the overlying Yellowstone caldera. The top of the chamber is about 8 km deep and the bottom is around 16 km deep. However, the chamber is not completely filled with fluid magma. It contains a partial melt, meaning that only a portion of the rock is molten (about 10 to 30%); the rest of the material is solid but, of course, remains hot.
The method that scientists use to discern this information is similar to medical CT scans that bounce X-rays through the human body to make three-dimensional pictures of internal tissue. In an analogous manner, a method called seismic tomography uses hundreds of seismograms to measure the speed of seismic waves from earthquakes and small, intentional dynamite explosions--data that allow geophysicists to make 3-D pictures of structures within the Earth. Scientists compare these seismic velocities, and infer the composition from deviations of these from average, thermally undisturbed values.